1. Field of the Invention
The present invention relates generally to an apparatus for controlling an automatic transmission of a motor vehicle, and more particularly to such an automatic transmission shift control apparatus for selecting an operating position of the automatic transmission, according to a shift control pattern, based on a value corresponding to a ratio of an engine intake air quantity to an engine speed.
2. Discussion of the Prior Art
There is known an electronic automatic transmission shift control apparatus for automatically controlling an automatic transmission of a motor vehicle, as disclosed in laid-open Publication No. 60-34563 of unexamined Japanese Patent Application. This shift control apparatus uses an intake air sensor for calculating an intake air quantity Q of an internal combustion engine of the vehicle. A value Q/N obtained by dividing the intake air quantity Q by a speed N of the engine, corresponds to a torque of the engine. The value Q/N will be referred to simply as "intake air quantity QN". The shift control apparatus is adapted such that the automatic transmission is automatically shifted up and down according to a shift control pattern representative of a relationship between the value Q/N (intake air quantity QN) and the running speed of the vehicle.
In this known automatic transmission shift control apparatus, the shift control pattern consists of shift-up boundary lines for shifting up the transmission, and shift-down boundary lines for shifting down the transmission. The shift control apparatus commands the transmission to be shifted up to a next high-gear position when the intake air quantity QN decreases below the level of an appropriate shift-up boundary line, and shifted down to a next low-gear position when the intake air quantity QN increases above the level of an appropriate shift-down boundary line. Thus, the shifting operation of the transmission is automatically controlled in response to a variation in the current load or currently required output of the vehicle engine.
The intake air sensor used for calculating the intake air quantity QN is usually disposed upstream of the engine, and is located a considerable distance away from a combustion chamber of the engine. Accordingly, there is a time lag between the moment of detection of an air flow by the intake air sensor, and the moment when the air flow as detected by the sensor reaches the engine combustion chamber, passing a throttle valve disposed between the sensor and the engine. Further, there is a certain volume of an air flow path between the throttle valve and the intake valve of the engine. This volume of the air flow path causes a difference between the intake air amount as calculated from the output of the intake air sensor and the actual amount of air currently flowing into the combustion chamber. Assuming, for example, that the throttle valve is rapidly opened for acceleration of the vehicle, at point of time t1 as indicated at (C) in FIG. 21, the amount of air flow at the position of the intake air sensor is a sum of the air quantity necessary to fill the volume of the above-indicated air flow path which has been at a considerably reduced pressure, and the air quantity which enters the combustion chamber. Consequently, the intake air quantity Q calculated from the currently obtained output of the intake air sensor is larger than the air quantity actually entering the combustion chamber, by an amount corresponding to the air quantity used to fill the volume of the air flow path between the throttle valve and the combustion chamber. As a result, the obtained intake air quantity QN as represented by solid line at (A) in FIG. 21 undergoes a sudden rise or overshoot, as indicated at I.
In the example of FIG. 21, the shift control pattern includes a first and a second shift-up boundary line U.sub.1, U.sub.2 indicated in one-dot chain lines at (A), for shifting up the transmission from the second-speed position to the third-speed position, and from the third-speed position to the fourth-speed position, and a first and a second shift-down boundary line D.sub.1, D.sub.2 indicated in dashed lines at (A), for shifting down the transmission from the fourth-speed position to the third-speed position, and from the third-speed position to the second-speed position. Suppose the transmission is currently placed in the fourth-speed position, the rapid increase in the throttle opening angle TA at point t1 causes the intake air quantity QN to exceed the levels of the first and second shift-down boundary lines D.sub.1 and D.sub.2, in this order, in a very short period of time (e.g., about 0.1 sec.), as a result of the overshoot indicated at I at (A). Consequently, the transmission is commanded to be shifted down by two steps, from the fourth-speed position down to the second-speed position. This two-step shift-down is unexpected to the vehicle driver, since the amount of increase in the throttle opening angle is not so large. Further, the transmission undergoes a shifting shock, leading to deterioration of the drivability of the vehicle. The shifting shock upon shifting of the transmission from the fourth-speed position to the second-speed position is particularly large, as compared with the other shift-down actions.
Since the overshoot I of the intake air quantity QN disappears in a very short time, the intake air quantity QN falls below the level of the first shift-up boundary line U.sub.1 immediately after the shift-down action, as indicated at (A) in FIG. 21. Therefore, the transmission is shifted up to the third-speed position as soon as the shift-down to the second-speed position is completed, as indicated at (B). This phenomenon gives the vehicle driver an impression of a busy shift of the transmission.
It is considered possible to eliminate the overshoot I by using a moving average or a weighted average of the intake air quantity QN (intake air quantity per revolution of the engine). This solution suffers from a delayed down-shift action when a rapid down-shift is required for fast acceleration of the vehicle.